1. Field of the Invention
The present invention relates to a communications antenna, and, more particularly, pertains to a slow wave structure in a slow wave antenna.
2. Description of the Prior Art
Historically, antenna elements of the prior art have been physically shortened below .lambda./4 (one-quarter wavelength), but as the elements are shortened below .lambda./4, the element radiation resistance becomes very low and the element exhibits capacitive reactance. In returning a shortened antenna element to electrical resonance, normally an inductive reactance is inserted in series along the length of the element. When the element is used as a simple .lambda./4 vertical antenna, the series tuning inductance can be positioned at the driving point, in the center, at the end opposite the driving point or distributed along the length of the element. The element is then referred to as being base loaded, center loaded, top loaded, or continuous loaded respectively depending on the positioning of the inductance.
Series inductance returns the shortened antenna element to resonance; however, in the case of an active or driven element, the radiation resistance reflected to the driving point still is quite low. In providing convenient matching to a transmission line, transmitter circuit, or receiver circuit, an impedance matching network is generally required. In any case, the inductively loaded antenna element exhibits a quite high Q and therefore has a quite narrow band width.
The Q is decreased and hence the bandwidth of an inductively loaded antenna element is increased by additional capacitance loading. This capacitance loading is generally accomplished by attaching disks, spikes, spokes or rings to the antenna element at or near the end opposite the driving point. The capacitance loading has the additional advantage of bringing the shortened element to resonance with less series inductive loading.
Practical prior art antennas incorporate physically shortened elements and utilize some type of series inductance and/or series-parallel capacitance loading to return the elements to electrical resonance at or near the operating frequency.
The present invention overcomes the disadvantages of the prior art by providing a slow wave antenna structure having unique and novel tuning and impedance adjustments in addition to having an element which is electrically .lambda./4 but physically less than .lambda./4.